1. Field of the Invention
The present invention relates to an image forming apparatus employing an electrophotographic process, and more particularly to an image forming apparatus such as a copying machine, a printer, a facsimile and the like.
2. Related Background Art
In the prior image forming apparatus, it has been proposed to use two fixing devices in combination to improve the fixing property of a toner image to a recording material whenever the recording material is of a large heat capacity such as a thick paper (Japanese Patent Application Laid-Open No. H06-258970).
However, in image forming apparatus utilizing two fixing devices in combination, since a recording material heated in a first fixing device is heated again in a second fixing device, there may result a change in a gloss of the toner image on the recording material, resulting from a change in the temperature of the recording material immediately before entering the second fixing device.
For example, in the case of an image forming job that executes a heating process in continuation on plural recording material bearing toner images with a first fixing device and a second fixing device, a temperature of the recording material changes immediately before entering the second fixing device, between a recording material in an early stage of the image forming job and a recording material in a latter stage.
In an early stage of the image forming job, the recording material heated in the first fixing device enters the second fixing device in a state somewhat cooled by a conveying roller, a conveying guide plate and the like for conveying the recording material from the first fixing device to the second fixing device, but, in a latter stage of the image forming job, the recording material heated in the first fixing device enters the second fixing device without such cooling because the conveying roller, the conveying guide plate and the like are in an already heated state. Therefore, the temperature difference immediately prior to the entry into the second fixing device becomes 50° C. or more between the recording material in the early stage of the image forming job and that in the latter stage. As a result, even within a same continuous image forming job, a gloss of the image on the recording material changes by about 5-10 between the early state and the latter stage of the image forming job. Such large change in the image gloss leads to a deterioration of the image quality in the continuous image forming job.